The proposed research project will identify molecular markers that are correlated with the clinical severity of toxoplasmosis in AIDS patients. Toxoplasma gondii is a common parasite of humans that chronically infects 15-90% of the world's population. Toxoplasma is also an important opportunistic pathogen in immunocompromised patients, causing life- threatening encephalitis in 10-15% of AIDS patients due to reactivation of chronic infections. Surprisingly, only 30% of AIDS patients who are chronically infected will develop toxoplasmic encephalitis, while the remainder do not develop disease. This variable pattern of reactivation is not simply due to the degree of immune dysfunction, as it is observed even in severely immunocompromised patients. Preliminary analysis, using a set of 6 independent PCR-RFLP markers, has demonstrated that a strong correlation exists between the parasite genotype and reactivation in AIDS patients. This pattern is particularly evident using the RFLP marker 62, which has a 95% correlation between the parasite genotype and reactivation in AIDS patients. The present study will expand the analysis of Toxoplasma strains isolated from AIDS patients to test the correlation of specific DNA markers with reactivation. A battery of PCR-RFLP markers will be used to determine the genotype of Toxoplasma strains isolated from AIDS patients relative to those found in animals or human congenital cases. Additional DNA markers will be developed by genetic mapping and PCR analysis. To identify molecular markers for reactivation, the genotype of strains will be correlated with the clinical severity of toxoplasmosis in AIDS patients. These studies will also identify genes involved in chronic pathogenesis of toxoplasmosis. The role of the parasite genotype and of specific genes will be tested in an animal model for reactivation, The cDNA marker 62, which is correlated with reactivation, will be used to develop serological reagents capable of detecting chronic infection with specific strains of Toxoplasma. These serological reagents will be evaluated for their ability to predict the likelihood of reactivation in AIDS patients. Development of these reagents should greatly aid in the clinical management and prevention of toxoplasmosis in AIDS patients.